Besides firing on the United States flag and personnel of the Unites States Army.
I take the fact that you did not respond as an acknowledgement that what you first said was incorrect, and now since I have made you aware of the fact that there was no pressing intent or desire to attack the fort was in fact the truth at that time, you no longer wish to press the point.
The evidence clearly shows that the arrival of those ships changed the equation. Note that it did specify that there would be no need to attack *IF* Anderson would agree to not fire on them.
That is clearly a reference to the anticipated confrontation between the Ships and the Confederate shore batteries.
Anderson of course did not agree, and he informed them that if they fired at those ships, he would fire at them.
This made it a military necessity to deal with him before the ships had a chance to assemble and organize their attack. Had he simply said that he would not attack them, they would have left him alone.
He told them that he would attack them, and this left Beauregard no choice and no wiggle room.
Had Anderson given his word, there probably would have been no attack on anyone, because unbeknownst to the Confederates, those ships were never going to attack them because Lincoln had sent the command ship to Florida under secret orders, where it immediately tried to attack Confederates along the shores, and did in fact fire shots at confederate ships.
As if it was deliberately trying to start a war at Ft. Pickens.